A recent development in communications technology is the transmission of data along standard power lines, known as broadband over power lines (BPL) or power line communication. The use of power line communications poses a number of challenges, including that power lines are inherently a very noisy environment. Every time a device is activated or turned off, noise (a pop or click) is added to the power line. Energy-saving devices within a home may introduce noisy harmonics into a power line. Therefore a communications system using power lines must accommodate the natural signal disruptions present.
Nearly all large power grids transmit power at high voltages across cities or between cities and at medium voltages between and across neighborhoods, to reduce transmission losses, using step-down transformers to reduce the voltage when approaching the end user. As BPL signals cannot readily pass through transformers, repeaters are attached to the transformers. In the U.S., typically a small transformer is hung from a utility pole to service a small number of (for example, 1 to 10) houses or homes (in some cases, a single house or even only street lights may be serviced with a transformer). In Europe, a transformer will typically service a larger number of houses.
Another issue in BPL is signal strength and operating frequency. BPL systems are expected to use frequencies in the 1 to 35 MHz range, which is commonly used by licensed amateur radio operators, international shortwave broadcasters and other communication systems (military, aeronautical, etc.). Power lines are unshielded and act as transmitters for the signals they carry, and thus may disrupt the 1 to 35 MHz range for such communications purposes.
Attempts to deal with the above challenges include U.S. Pat. No. 6,452,482 to Cern. The '482 patent discloses a coupler for coupling a data signal to a conductor in a medium voltage power transmission cable.
U.S. Pat. No. 6,809,633 to Cern discloses a coupling broadband modem to power lines. The '633 patent discloses the use of couplers attached to neutral lines for loopback purposes in low voltage BPL communications.
U.S. Pat. No. 7,091,849 to Henry discloses a method and apparatus for reducing inbound interference in a broadband power line communication system that adjusts the phase or amplitude of one of first or second carrier signals.
U.S. Pat. No. 7,088,232 to Wetmore discloses a system and method for reducing radiation when distributing broadband communication systems over power lines.
FIG. 1 shows a traditional priori art communications system for power grids, with signal transmission occurring on MV and LV power lines, using couplers 118 and 120 to connect transmitter 112 and receiver 124, respectively, to the power lines, and being subjected to RF noise problems, safety concerns, and reliability/durability problems. Utility poles include three separate sections, one for distribution for medium voltage (MV) power, a utility space for low voltage (LV) power and a telecom space. Traditional BPL equipment for utility pole to utility pole transmission uses MV lines, and therefore relies on the MV distribution section of the utility pole. Utility companies often require a very lengthy approval process (for safety and reliability, durability, etc.) to ensure equipment placed in this section of the utility pole does not negatively affect power on the grid or cause safety problems.